This invention relates to the control of emissions from fossil-burning power plants, and, more specifically, to an electrostatically-stimulated fabric filter utilized in a flue gas treatment system.
Flue gas treatments used in conjunction with fossil-fuel power generation plants often employ fabric barrier filters to remove particulates from the flue gas before the gas is exhausted through a stack to atmosphere. Fabric barrier filters include baghouses of the pulse-jet type and reverse flow or shake-deflate type for periodically removing the dust cake accumulated on the surface of the bag filter. The fabric bag filters typically have a permeability of 25 to 50 acfm/sq.ft (actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water or greater). Examples of conventional fabrics used in such filters include PPS and TEFLON®/glass. These fabrics, however, can experience high pressure drop problems when applied at high air to cloth ratios, when installed downstream of an electrostatic precipitator, or when experiencing heavy inlet dust burden. Fans are typically employed to overcome the additional pressure drop required to draw the flue gas across the barrier filter. The operating cost of a fabric filter is heavily dependent on the system pressure drop experienced. Operating costs associated with a fabric filter can be reduced by lowering system pressure drop. The number of cleaning cycles imposed on a fabric filter impacts expected useful life of the bag. In addition, if the fabric filter system can operate with a reduced number of cleaning cycles, bag life can be extended.
High-permeability fabric filter bags have also been used and generally reduce pressure drop. A high-permeability fabric filter bag is disclosed in U.S. Pat. No. 6,514,315 that is said not to have the high pressure drop problems associated with conventional fabric filters. High permeability fabric filters have permeabilities higher than 76 and up to 200 acfm/sq.ft. Of concern, however, is the higher emissions experienced with such high permeability fabrics.
In another recent development, a dust collection system combines Discharge Electrodes and fabric filters in the same casing. This arrangement has been characterized as an electrostatic precipitator that uses fabric filters instead of collection plates. Such electrostatically-stimulated filters have been used as slipstream units to augment existing dust collectors, as polishing units behind existing precipitators, baghouses or scrubbers, or as a stand-alone high efficiency precipitator. To the best of our knowledge, these units use only conventional fabric filters.
There remains a need, therefore, to create a filter barrier of high permeability for reduced pressure drop, but that does not also result in higher emissions.